What do you expect to happen here? We just don't have unboxed ints at the type level. Doubtless the error message could be improved.

Well, a message to that effect would be much better than what we get now! But I don't have a sufficiently clear sense of exactly what is and is not allowed to write it myself. Something about variables bound in type families not being able to have kinds of kind TYPE r unless r ~ PtrRepLifted, perhaps?